While the invention is subject to a wide range of applications, it is especially suited for use in a hydraulic circuit required in a vehicle and will be particularly described in that connection. In hydraulic systems for vehicles, such as, for example, a refuse truck, the general practice is to have a hydraulic circuit for performing work functions such as, for example, compressing the refuse or raising the back to unload. The circuit many include an actuator which is powered by fluid delivered from a fixed displacement pump being operated by the main engine of the vehicle.
When the vehicle is not moving, the engine may idle at approximately 1000 revolutions per minute (rpm) and the pump is geared to a drive shaft from the engine such that the pump may rotate at a higher speed, such as, for example 3000 rpm. When the vehicle starts to move, the engine begins operating at a higher speed, such as, for example, 3000 rpm and the pump speed correspondingly increases to approximately 9000 rpm.
When the pump is operating at such a high speed, it is displacing a large quantity of hydraulic fluid. This fluid is not needed when the hydraulic functions are not being performed, such as when the vehicle is moving. Since the fluid acts against a closed actuator valve, the pressure in the system drastically increases. The pressure may be relieved by a relief valve to a reservoir. The flow of hydraulic fluid across a pressure drop as created by the relief valve causes a horsepower loss. The resulting loss in efficiency means that additional fuel is required to operate the engine of the vehicle. Since fuel may be expensive, minimizing inefficiency is very important.
Another problem with running the pump at high speed is that it creates a great deal of noise. Since a refuse truck is operated in a residential area, the noise factor is important as it may be quite disturbing to the residents of the area.
Another consideration in operating a pump at a high speed and against a heavy load is that it may wear out in a shorter period of time. Increasing the frequency of pump repair adds to the expense of operating the equipment and may cause costly time delays when the vehicle is being repaired.
It is an object of the present invention to provide a hydraulic system which has a relatively low horsepower loss.
It is a further object of the present invention to provide a hydraulic system which has a quiet operation.
It is a further object of the present invention to provide a hydraulic system which has increased reliability.
It is a further object of the present invention to provide a hydraulic system which is relatively inexpensive to operate.